1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor (hereinafter, also referred to as a photoreceptor) of laminated type and single layer type having a photosensitive layer containing an organic material, which is used in electrophotographic devices such as printers, copying machines and facsimiles employing an electrophotographic system, a process for producing the electrophotographic photoreceptor, and an electrophotographic device mounted with the photoreceptor.
2. Background of the Prior Art
Electrophotographic photoreceptors are required to have a function of retaining surface charges in the dark, a function of receiving light and thereby generating electric charges, and a function of similarly receiving light and thereby transporting electric charges. Examples of such electrophotographic photoreceptors include so-called laminated type photoreceptors in which functionally separated layers such as a layer that contributes mainly to the generation of charges and a layer that contributes to the retention of surface charges in the dark and to the transport of charges upon light reception, are laminated; and so-called single layer type photoreceptors in which a single layer combines these functions.
In the formation of images according to an electrophotographic method using these electrophotographic photoreceptors, for example, Carlson's process is applied. The formation of an image by this system is carried out through electrostatic charging of a photoreceptor in the dark, formation of an electrostatic latent image on the surface of the charged photoreceptor under the effect of exposure in accordance with the characters or drawings in the manuscript, development of the formed electrostatic latent image using toner, and transfer and fixation of the formed toner image onto a support such as paper. After the transfer of the toner image, the photoreceptor is subjected to the removal of residual toner, charge elimination and the like, and then is provided for reuse.
Some of the electrophotographic photoreceptors described above make use of an inorganic photoconductive material such as selenium, a selenium alloy, zinc oxide or cadmium sulfide. In recent years, organic photoreceptors in which an organic photoconductive material that is advantageous in terms of thermal stability, film-forming properties and the like as compared with the inorganic photoconductive materials, is dispersed in a resin binder, have been brought to practical application and now constitute the mainstream. Examples of such an organic photoconductive material include poly-N-vinylcarbazole, 9,10-anthracenediol polyester, pyrazoline, hydrazone, stilbene, butadiene, benzidine, phthalocyanine, and bisazo compounds.
Among the organic materials that are used in these organic photoreceptors, the organic photoconductive materials which are in charge of the function of charge generation and the function of charge transport, are in many cases low molecular weight materials with less ability to form layers, and thus it has been difficult to form a photosensitive layer having durability. However, it has been made possible to produce an organic photoreceptor having a photosensitive layer with high durability and practical film strength, by subjecting such a low molecular weight material to primary dispersion or dissolution in a high molecular weight compound with greater ability to form layers (resin binder), and then forming a photosensitive layer.
Recently, the functionally separated laminated type photoreceptors described above, in which a charge generation layer containing a charge generating material and a charge transport layer containing a charge transporting material are laminated as photosensitive layers, are constituting the mainstream because, based on the rich variety of organic materials, a wide selection of materials appropriate for the various functions of the photosensitive layers allows a large degree of freedom in design.
Among others, negatively charged type photoreceptors in which a charge generation layer containing a photoconductive organic pigment is formed on an electroconductive substrate and a charge transport layer containing a charge transporting material is laminated on the charge generation layer, are now available as a variety of commercial products. Usually, this charge generation layer is formed into a film by vapor deposition of a photoconductive organic pigment, or is formed into a film by immersion coating from a coating liquid in which a photoconductive organic pigment is dispersed in a resin binder, and the charge transport layer is formed by immersion coating from a coating liquid in which a low molecular weight organic compound having a charge transport function is dispersed or dissolved in a resin binder.
Furthermore, positively charged type photoreceptors which use a single layer of photosensitive layer in which a charge generating material and a charge transporting material are all dispersed or dissolved in a resin binder, are also widely known.
When an electrophotographic photoreceptor to an electrophotographic device of Carlson's process system, the following matters frequently constitute problems to be solved.
(1) To improve adhesiveness between the photosensitive layer and the electroconductive substrate.
(2) To increase concealability against defects of the substrate surface or surface unevenness.
(3) To suppress the generation of defects such as black dots or white dots on a printed image, that are caused by unnecessary carrier injection from the electroconductive substrate.
Thus, in order to solve the problems of (1) to (3), it is known to insert an undercoat layer between the substrate and the charge generation layer of a laminated type photoreceptor or the photosensitive layer of a single layer type photoreceptor. As this undercoat layer, a layer of a resin such as a polymeric compound, or an anodic coating is conventionally used.
When the undercoat layer is formed from a resin such as a polymeric compound, it is known that the usage of a thermoplastic resin such as polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, polyester or polyamide, or of a thermosetting resin such as an epoxy resin, a urethane resin, a melamine resin or a phenolic resin, as the constituent material is under investigation, for example, Japanese Patent Application Laid-Open (JP-A) No. 52-100240 (Patent Document 1), JP-A No. 58-106549 (Patent Document 2), JP-A No. 54-26738 (Patent Document 3), JP-A No. 52-25638 (Patent Document 4), JP-A No. 53-89435 (Patent Document 5), and the like.
There is known an undercoat layer which is prepared by further dispersing metal oxide fine particles, and which therefore does not cause a significant decrease in sensitivity even if prepared into a thick film, while maintaining concealability against defects of the substrate surface. Furthermore, an undercoat layer which is prepared by dispersing organic compound-treated metal oxide fine particles and thereby exhibits effectiveness in electrical properties, is also already known, for example, Japanese Examined Patent Application (JP-B) No. 2-60177 (Patent Document 6), Japanese Patent No. 3139381 (Patent Document 7), and the like.
In addition, investigations have been hitherto conducted on various polymeric compound resins for their use in an undercoat layer which generally focuses on the countermeasures against memory generation that occurs in a low temperature and low humidity environment in which the undercoat layer attains high resistance, and the countermeasures against the occurrence of black dots or the occurrence of fogging defects in printed images in a high temperature and high humidity environment in which the undercoat layer attains low resistance. For example, JP-A No. 2002-6524 (Patent Document 8) discloses a mixture in which melamines and guanamines are applied as crosslinking agents to a polyester resin.
It is also reported in JP-A No. 2007-178660 (Patent Document 9) that when a resin containing a dicarboxylic acid and a diamine as constituent monomers at a defined composition ratio is applied, image characteristics that are satisfactory for all environments ranging from low temperature and low humidity environments to high temperature and high humidity environments, can be obtained.
Furthermore, there have been suggested attempts to solve the problem of light-induced fatigue by an improvement of the undercoat layer (intermediate layer). For example, JP-A No. 8-262776 (Patent Document 10) discloses an electrophotographic photoreceptor which contains an organometallic compound, a coupling agent and the like in the undercoat layer, and contains inorganic fine particles in the surface layer. JP-A No. 2001-209201 (Patent Document 11) also discloses an electrophotographic photoreceptor which uses an azo pigment and a phthalocyanine-based pigment as charge generating materials, and contains titanium oxide and a metal oxide in the undercoat layer. In these patent documents, descriptions on the effect on light-induced fatigue due to repeated use or on pre-exposure fatigue can be found. Furthermore, JP-A NO. 5-88396 (Patent Document 12) discloses a photoreceptor which includes an undercoat layer containing hydrophobic silica fine particles for the purpose of obtaining satisfactory images.
However, in the photoreceptors which use the above-described materials such as those described in Patent Documents 1 to 12 for the undercoat layer, the resistance of the undercoat layer varies with the changes in temperature and humidity. For that reason, when such photoreceptors are mounted in recent electrophotographic devices where high quality of images is demanded, there is a tendency that it is not easy to simultaneously attain the electric potential characteristics that are stable in all environments ranging from low temperature and low humidity environments to high temperature and high humidity environments, and the image quality in a satisfactory manner.
Furthermore, along with the development of color printers and a rise in the distribution rate thereof in recent years, an increase in the printing speed or a reduction in size or component-count of the device is in progress, so that measures to cope with various use environments are also in demand. Color printers have a tendency that the transfer current increases as a result of transfer with toner color overlap or employment of a transfer belt. Therefore, in the case of performing printing on papers of various sizes, there occurs a difference in the fatigue due to transfer between the areas with paper and the areas without paper, and this causes a failure in which differences in the image density is promoted. That is to say, if printing has been performed more frequently on small-sized paper, in contrast with the part of photoreceptor where paper is present (paper passing area), the part of photoreceptor where paper does not pass (non-paper passing area) is continuously subjected to direct influence of transfer, so that the fatigue due to transfer increases. As a result, when printing is performed on large-sized paper next time, the difference in the fatigue due to transfer between the paper passing area and the non-paper passing area brings on a problem that a potential difference occurs in the developed area, causing a difference in density. This tendency becomes more conspicuous when there is an increase in the transfer current. Furthermore, there are an increasing number of situations in which, when the cover of a printer is opened due to problems such as a paper jam or cartridge exchange, the photoreceptor is left in exposure to light. As a result, there is a density difference even between the light-exposed area and the non-light-exposed area, and thus the problem with the emergence of light-induced fatigue is becoming serious. Under such circumstances, in contrast with monochromatic printers, the demand for reliability in photoreceptors, such as transfer restorability or restorability from intense light-induced fatigue, is markedly increasing particularly in color printers. However, conventional photoreceptors have not been able to meet these demands simultaneously and sufficiently.
Furthermore, in Patent Document 8, there is no description on the investigation on possible application of copolymer resins for which the constituent monomers of the resins or the composition ratios of the monomers are not sufficiently defined. Therefore, although effects are shown in connection with the electric potential characteristics or image quality in high temperature and high humidity environments, the invention cannot be expected to have effects on the potential characteristics that are stable in all environments ranging from low temperature and low humidity environments to high temperature and high humidity environments.
In regard to Patent Document 9, it is the actual situation that sufficient investigations have not been conducted on the restorability from intense light-induced fatigue and restorability from fatigue due to transfer.
Patent Documents 10 and 11 have descriptions that effects on light-induced fatigue due to repeated use, or effects on pre-exposure fatigue can be expected. However, reports on the investigation focusing on the restorability from intense light-induced fatigue and restorability from fatigue due to transfer, and the possibility of achieving a good balance therebetween, are hardly found. That is, photoreceptors that use the undercoat layers that have been hitherto investigated can be put to practical use in monochromatic printers, which do not seem to have problem with the restorability from fatigue due to transfer or with the restorability from light-induced fatigue; however, there is a problem that it is difficult for the photoreceptors to be adapted to color printers where these properties are demanded at a high level. This problem would become more significant, since even color printers also have a tendency that the transfer current increases as the printing speed increases. Particularly, the problem will become more noticeable when the printing speed is 16 ppm (A4, vertical) or greater.
In addition, Patent Document 12 discloses a photoreceptor which includes an undercoat layer containing hydrophobic silica fine particles. Furthermore, a description on a polyester amide resin as the resin for the undercoat layer, is found in paragraph of Patent Document 12. However, in the Patent Document 12, sufficient investigations have not been conducted on the storability from intense light-induced fatigue and the restorability from fatigue due to transfer. Particularly, there is no clear description on whether the effects of the restorability from intense light-induced fatigue and the restorability from fatigue due to transfer can be obtained with all kinds of polyester amide resins.
Thus, the present invention was made in view of the problems described above, and an object of the present invention is to provide an electrophotographic photoreceptor which includes an undercoat layer capable of attaining electric potential characteristics that are stable in all environments ranging from low temperature and low humidity environments to high temperature and high humidity environments, and of suppressing the occurrence of printing defects. Another object of the present invention is to provide an electrophotographic photoreceptor which includes an undercoat layer that is capable of simultaneously attaining the transfer restorability and the restorability from intense light-induced fatigue even in a wide variety of usages and operation environments, and which is consequently capable of printing satisfactory images in which image defects or density differences do not easily occur. Still another object of the present invention is to provide a process for producing the photoreceptor, and an electrophotographic device mounted with the photoreceptor. That is, the present invention is intended to provide an electrophotographic photoreceptor from which sufficient effects can be expected as built-in performances in high speed color printers, a process for producing the photoreceptor, and a color printer mounted with the photoreceptor.